Numerous radar and communications applications urgently require compact, broadband and efficient true-time delay technologies. In the context of radar applications, large fixed radiofrequency (RF) delays are critical for the operation of RF tags, while large arrays of tunable delays are needed to replace mechanically steerable antennae with electronically steerable antennae. In the optical domain, large optical delays are needed to buffer data during the switching and routing of data.
Radar identification using RF tags uses large fixed delays in order to utilize the same antenna element to perform both receive (Rx) and transmit (Tx) functions. Through RF tag identification, the tag must take in a radar signal, delay or buffer it for a period of at least 100 ns, and encode data onto it before transmitting the pulse. In this case, the delay-line enables Rx and Tx functions to be performed through use of the same antenna by switching the mode of operation at different points in time. The key challenge in the case of tag technologies is to keep the size and power to a minimum if such devices are to remain unnoticed. Ideally, the entire device is about the size of a credit-card.
Currently, large RF delays are implemented through use of surface-acoustic-wave (SAW) delay-lines. While SAW delay technologies are very stable and mature, state-of-the-art SAW delay-lines have relatively low operating frequencies (˜1 GHz) and are bandwidth limited to ˜500 MHz. In addition, SAW devices typically exhibit very high RF insertion losses (˜36 dB). This poses a problem, as the radar signals that tags must process are generally centered between 8-15 GHz. Therefore, in order to make use of the large delays afforded by SAW delay-lines, current tag architectures must down-convert the RF signals that they wish to delay and encode through use of a mixing operation. This results in a 10× increase in the number of components, and generates a significant increase in power consumption (generally adding ˜400 mW of power consumption).
In addition to RF delay, compact all optical delay lines are needed for data switching, routing and network synchronization of high speed (10-100 GBs) data networks (e.g., to enable header decoding, frame construction or elastic store functions). Both fixed and variable optical delays of (˜100 ns) are required while maintaining low insertion loss at high data rates. The primary challenge faced in generating optical delays of this magnitude is posed by optical waveguide losses. Currently, planar lightwave circuits (PLCs) are used to achieve low losses; however, state-of-the-art insertion losses using this technology are currently ˜30 dB. Furthermore, the length of such PLC waveguides tends to be ˜20 meters, which places a lower limit on the size of such devices.
However, a single chip-scale technology that offers ultra narrowband filtering, large signal delays (μs-ms), and the ability to integrate complex processing of RF signals (e.g., correlators, switches etc) over broad bandwidths (GHz) remains elusive, while the need for such technologies in the context of RADAR, LADAR, communications, and transponder applications continues to grow. See R. Williamson and R. Esman, J. of Lightwave Technology 26 (9), 1145 (2008). A host of technologies and physical processes have been explored to address these technological challenges, with limited success. In the context of RF-photonics, numerous chip-scale optical slow-light solutions have been proposed through which one seeks to replace spools of fiber used for delay with chip-scale waveguides. See F. Xia et al., Nature Photonics 1 (1), 65 (2006); and T. Tanabe et al., Nature Photonics 1 (1), 49 (2006). However, fundamental optical losses make retention of signals for times in excess of 1 ns difficult with chip-scale optical waveguides. MEMS switching technologies for switching RF transmission line arrays and steering of free-space mirror arrays have also been explored. See B. Anderson et al., Proceedings of SPIE 7669, 76690P (2010); and C. Nordquist et al., IEEE Microwave and Wireless Components Letters 16 (5), 305 (2006). These technologies continue to face fundamental reliability and speed limitations. Finally, while the favorable loss characteristics of phononic media make electromechanical MEMS resonators and FBAR resonators attractive for signal delay, severe bandwidth and transduction efficiency limitations of electromechanical transducers make such technologies unsuitable for today's high bandwidth needs. See E. Dieulesaint and D. Royer, Elastic waves in solids II: Generation, acousto-optic interaction, applications, Springer (2000); and D. Morgan, Surface acoustic wave filters: with applications to electronic communications and signal processing, Academic Press (2007).
In the context of signal processing technologies, phononic signals are unique for their ability to preserve coherence for remarkably long times (milliseconds), transmit and process information over ultra-high bandwidths (10-100 GHz), and carry signals via sub-micron wavelengths—allowing for dramatic size reductions over conventional radio frequency (RF) processing components. Development of new phononic domain signal processing technologies, utilizing these advantages, enables thousands of ultra-high performance devices for delay and filtering to be integrated on centimeter-scale chips. To date, however, no mechanisms for broadband phononic signal amplification and transduction, necessary for the realization of such technologies, has been developed. See R. Weigel et al., IEEE Transactions on Microwave Theory and Techniques 50(3), 738 (2002); E. Dieulesaint and D. Royer, Elastic waves in solids II: Generation, acousto-optic interaction, applications, Springer (2000); D. Morgan, Surface acoustic wave filters: with applications to electronic communications and signal processing, Academic Press (2007); and R. Williamson and R. Esman, J. of Lightwave Technology 26(9), 1145 (2008).
Acoustic transduction via optomechanical photon-phonon coupling has caught the attention of many since nanoscale light confinement produced remarkably large forces within miniscule volumes, and it is known to produce high frequency phonon transduction. See E. Ippen and R. Stolen, Applied Physics Letters 21(11), 539 (1972); R. Chiao et al., Physical Review Letters 12(21), 592 (1964); R. Shelby et al., Physical Review B 31(8), 5244 (1985); G. Agrawal, Nonlinear fiber optics, Academic, San Diego, Calif., (1995); M. Eichenfield et al., Nature 459, 550 (2009) and M. Eichenfield et al., Nature Photonics 1(7), 416 (2007). To date, however, only narrow-band transduction (˜10 MHz) has been demonstrated at modest powers by optomechanical cavity systems and resonant stimulated Brillouin scattering (SBS). Simultaneous optical and mechanical resonances of such systems result in large optomechanical coupling, albeit at an inherent cost in bandwidth. While GHz-data-rate signal processing may, in principle, be accommodated by thousands of resonant optomechanical cavities at equally-spaced frequencies, fundamental challenges—surrounding sophisticated encoding and decoding schemes, large device area, and stringent fabrication tolerance—must be addressed. Furthermore, while SBS might be proposed as an alternate solution, limited photon-phonon coupling in conventional materials render such processes impossible in chip-scale silicon photonics technologies. See B. Jalali et al., IEEE Journal of Selected Topics in Quantum Electronics 12 (6), 1618 (2007).
Therefore, a need remains for a chip-scale technology that can meet the current delay-line challenges for use in RF or optical signal processing applications.